Broadly tunable continuous-wave laser using color centers

ABSTRACT

Broadly tunable infrared lasers analogous to dye lasers operating in the visible spectrum are provided by significant improvements upon an F A  (II) color center laser previously demonstrated in a limited way. The improvements include techniques for also using F B  (II) and F 2   +  color centers and include substantially increased concentrations of the F-centers in regions of pumpable geometry, judicious choice of pumping frequencies and powers and variable, frequency-selecting resonators that are capable of producing oscillation anywhere in the color center fluorescence bands. All solid-state cooling by means of contact between the crystal and a solid heat sink is provided in a way that provides the optical quality needed for efficient operation and for greatest tuning bandwidth of the laser. Use of the color centers in distributed feedback devices is described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending patentapplication, Ser. No. 438,200, filed Jan. 31, 1974, assigned to theassignee hereof, and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to coherent radiation sources of the type thatare capable of continuous-wave, broadly tunable or pulsed operation andparticularly such systems having those characteristics in the infraredportion of the optical spectrum.

The advent of the dye laser has produced substantial changes both in theapproach to study of materials and in applied research for futureoptical display and data processing systems and, more distantly, forfuture optical communication systems. Because of the tunability of somedye lasers throughout the visible portion of the spectrum, appliedresearch efforts on nonlinear optical coherent sources for tunableradiation in the visible spectrum have been de-emphasized.

Nevertheless, in the laboratory study of new materials and similarspectroscopy uses, tunable infrared coherent radiation is still providedby tunable nonlinear optical oscillators. Those nonlinear opticaloscillators are extremely complex, are difficult to adjust and operateand are very costly. At present, there is no broadly acceptedalternative in the nature of a dye laser for the infrared portion of thespectrum.

BRIEF SUMMARY OF THE INVENTION

According to my invention, the need for a cheaper, more easily usedsource of tunable infrared radiation is satisfied by a color centerlaser incorporating substantial improvements over the only previouslydisclosed color center laser, the improvements including techniques forusing F_(B) (II), F₂ ⁺ and F_(A) (II) color centers.

For any of these color centers, primary features of my invention includeparticularly an arrangement for solid-state cooling of the activemedium, and appropriate pumping frequencies, polarizations and powersand variable, frequency-selective resonators. Solid-state cooling iscooling by conduction to a contacting solid body.

It is one advantage of my laser that its extremely broad usablebandwidth, which is made continuously available by the foregoingimprovements, can also be used for the generation of extremely shortinfrared pulses or for highly efficient fixed single-frequencyoscillations.

It is one aspect of my invention that the foregoing improvements arebased in large part on my recognition of the possibility, and on mydefinitive experimental demonstration, that such a color center laserhas a truly homogeneous broadened laser line, whereas the prior art leftlittle promise of this with its ambiguous speculations and contradictoryassertions. For example, see the article by B. Fritz et al, Solid-StateCommunications, Volume 3, page 61 (1965) and Chapter 3 written by F.Luty in the book Physics of Color Centers edited by W. B. Fowler,Academic Press, New York (1968). The latter merely notes inconsistencywith the common type of inhomogeneous broadening, but sees theappearance of satellite lines as discouraging with respect tohomogeneous broadening. In other words, despite the need for an infraredequivalent of the dye laser, those prior teachings did nothing toencourage attention to application of color centers or to color centerlasers, in particular.

Another aspect of my invention is based upon my discovery of thepossibility of broadly tunable, efficient continuous-wave laser actionthroughout the range from about 0.9 μm to about 3.3 μm by employingF_(A) (II) centers caused either by lithium or sodium as dopingimpurities, F_(B) (II) centers and F₂ ⁺ centers. This aspect of myinvention in my parent application involved the use of F_(A) (II):Licenters, as set out in my above-identified parent application.

Further advantages reside in the apparent absence of bleaching or agingeffects, in appropriate configurations, quite in contrast to theintractable bleaching in organic dyes. Also, the threshold optical pumppower is quite low, on the order of 30 times less than that required forthe most efficient dye lasers.

It is one feature of my invention that it is made compatible with futureintegrated optical circuits by its use of mainly solid-state cooling.The active medium can thus be deposited directly on a substrate withsubstantially similar lattice constants. In the versions more amenableto a laboratory setup, for example for spectroscopy, a weak spring clipprovides a slip-fit mounting of an active platelet to a cold fingerthrough which a hole is provided to allow the pumping beam to passthrough the active platelet at the Brewster's angle.

Another feature of my invention is based on the recognition that thepump band of the F_(B) (II) centers overlaps that of the interferingF_(A) (I) and F_(B) (I) centers, which always coexist with the F_(B)(II) centers, but that the type I centers can be effectively "bleached"or rendered non-interfering by aligning them both along a common {100}axis by strong pumping by polarized light when the crystal is at or nearroom temperature. The F_(B) (II) centers are then pumped with lightpolarized at right angles to the {100} axis.

According to still another feature of my invention, the F₂ ⁺ center isemployed by pumping only the lowest energy absorption band. This pumpingresults in the highest energy conversion efficiency and, moreimportantly, avoids the creation of excited species that would reabsorbthe emitted F₂ ⁺ radiation.

According to more specific features of my invention, the concentrationof F_(A) (II) centers, in cases using those centers, is made to be atleast 1 × 10¹⁷ centers per cubic centimeter; and the pumping source isselected to match one of the broad absorption bands of the colorcenters. If a matching pump source is available, the lowest frequencyband is preferred. For F_(A) (II) color centers in potassium chloride(Li^(+-doped)) this lowest-frequency absorption band is in the redportion of the visible spectrum.

With the provision of all the foregoing features in a color center laserusing a lithium-doped potassium chloride (KCl) crystal, continuous-wavelaser action has been demonstrated and has been continuously tuned overthe wavelength range from 2.5 to 2.9 micrometers. Laser action wasobtained for crystal temperatures up to about 200°K.

In an improvement of an F_(A) (II) center laser according to myinvention, the active crystal is oriented with its {110} axis at anacute angle to the pump beam direction and lying in the plane defined bythe pump beam direction and pump beam polarization, in order to pumpcenters of all three orthogonal orientations, {001}, {010} and {100},and thereby avoid bleaching effects.

Advantageously, the form of resonator for these new lasers has now beenshown to be unimportant, except for the provision of solid-statecooling. Thus, one may employ either the folded resonator of U.S. Pat.No. 3,731,224 issued May 1, 1973 to A. Dienes et al., distributedfeedback technique of U.s. Pat. No. 3,760,292 issued Sept. 18, 1973 toH. W. Kogelnik et al., as well as those shown in my above-citedcopending parent patent application and many others.

BRIEF DESCRIPTION OF THE DRAWING

Further features and advantages of my invention will become apparentfrom the following detailed description, taken together with thedrawing, in which:

FIG. 1 is a partially pictorial and partially block-diagrammaticillustration of the basic embodiment of my invention;

FIGS. 2A and 2B show two views of the slip-fit mounting of the activecrystal on an appropriately-arranged cold finger;

FIG. 3 shows a proposed modified embodiment of my invention compatiblewith integrated optical circuits;

FIG. 4 shows an energy level diagram useful in describing the theory andoperation of my invention;

FIG. 5 shows curves of luminescent intensity versus wavelength fordiffering alkali halides and types of color centers usable according tomy invention;

FIGS. 6A and 6B show schematically the normal and relaxed configurationsof an alkali halide with F_(A) (II) color centers, making possible theenergy level diagram of FIG. 4;

FIG. 7 is a schematic illustration of the crystalline latticearrangement for an F₂ ⁺ color center;

FIG. 8 shows the F₂ ⁺ energy level diagram;

FIGS. 9A and 9B are schematic illustrations of the normal and relaxedconfigurations of an alkali halide with F_(B) (II) color centers;

FIG. 10 is a partially pictorial and partially schematic illustration ofa modification of the resonator of FIG. 1, with unchanged portions notshown;

FIG. 11 is a partially pictorial and partially schematic illustration ofa more detailed modification of the resonator of FIG. 1;

FIGS. 12-14 shown curves useful in explaining the operation of theinvention; and

FIG. 15 shows a partially pictorial and partially block diagrammaticillustration of another embodiment of the invention employingdistributed feedback.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The first four figures are as shown in my above-cited parent patentapplication.

An experimental laboratory version of my invention essentially accordingto the pictorial diagram shown in FIG. 1 was used to demonstratecontinuous-wave laser action in a lithium-doped (Li⁺) potassium chloride(KCl) crystal containing F_(A) (II) color centers. The crystal 11 wasmounted on a copper cold finger 14 at the Brewster's angle with respectto the pumping beam from a Krypton ion laser 18 operating at 6471 A andwas disposed in an optical resonator, including the input mirror 12which passed the pumping beam and an output mirror 13 which passed thestimulated emitted coherent radiation. Also an integral part of theresonator was the low-loss optical path provided through the cold finger14, that is, channel 31 adjacent the central portion of crystal 11. Ahighly transparent infrasil lens 25 and the diffraction grating 26,which directed a substantial portion of the first order beam of thestimulated coherent radiation back toward the crystal 11, formed anadditional resonator with input mirror 12. Lens 25 and grating 26 couldalso be viewed just parts of one compound resonator with mirrors 12 and13; their role was particularly to facilitate tuning. The remainingportion 28 of the first order beam was received in a desired utilizationapparatus 29. In actual laboratory usage, the apparatus 29 would includea sample under study and a spectrometer; but, in my experiments, itincluded a high resolution monochrometer and an appropriate detector forpower measurements.

The crystal 11 was in the form of a platelet with its thin dimension,about 1 to 2 millimeters, traversed by the pumping beam and wasdisposed, together with the cold finger 14, mirrors 12 and 13 and theirassociated mounts in a vacuum chamber 15 into which the pumping beampassed through the infrared-transmitting window 16. The vacuum chamber15 was evacuated by the mechanical pump 23 which included a cold trap ofconventional type. The cryogenic reservoir 20 was an involuted portionof the exterior of vacuum chamber 15 exposed to the atmosphere through anarrow neck 30 which provided a minimal rate of vaporization of thecoolant, typically liquid nitrogen. The vaporization boiling of thecoolant maintained the cold finger 14 and crystal 11 at the selectedoperating temperature. By choosing various coolant media, severaloperating temperatures were made possible in particular 77°K and 200°K.For example, one such different coolant is a slurry of dry ice andacetone.

Adjustability of the position of output mirror 13 relative to inputmirror 12 (for cavity alignment) was provided by the metal bellows 24.Cross tuning can be achieved solely by rotation of the diffractiongrating 26 about an axis parallel to its surface and orthogonal to thedirection of the incident laser light.

The laser crystals were prepared as follows: Platelets of the desiredthickness (t˜1-2 mm) were cleaved out of a KCl:Li crystal containingabout 10¹⁸ U-centers/cm³. U-centers are made by drifting hydrogenthrough a KCl crystal containing F-centers. The F-centers wereillustratively made by the well-known process of additive coloration, asexplained by C.Z. vanDoorn, Rev. Sci. Instr., 32, 755 (1961). Thehydrogen converts F-centers to U-centers. The two opposing large faces,{100} planes, of a platelet were then optically polished. Since theU-centers absorb only in the hard UV, the resultant clarity of thecrystal greatly facilitated inspection for surface flaws, strain-inducedbirefringence, and other possible defects. A fraction of the U-centerswere then converted into ordinary F-centers by subjecting the crystalfor about 10 min. to 50 kV X-rays, generated by a beam current of 17 ma,with the crystal located ˜2 cm from the anode. The resultant F-centerconcentration was about 1 - 2×10¹⁷ /cm³. Immediately following X-raytreatment, the crystal was loaded into the laser dewar and cooled toabout -30°C. The F-centers were then converted to F_(A) (II) centers byoptically pumping the F-center absorption band (in the green) for nearlyan hour, at an intensity, I = 0.1 W/cm². Finally, the crystal was cooledto 77°K for the laser experiment.

In my early experiments, the optical cavity defined by mirrors 12 and 13had a separation between the mirrors just a few millimeters shorter thanthe sum of the radii of the two mirrors, the sum being of the order of175 mm. For the wavelength range 2.5 ≦ λ ≦ 2.9μm the output mirrors 12and 13 had reflectivities R ≅ 100% and R ≅ 95%, respectively. Mirror 12had a transmission T > 80% at the pump wavelength 6471 A.

The slip-fit mounting of crystal 11 was found necessary, and no greaseor other thermally conductive compound was used, in order not to strainand thereby fracture the crystal by differential thermal expansion orcontraction. The crystal 11 was oriented such that the plane common tothe laser beam and normal to the crystal faces contained a {110} axis,primarily to avoid affecting the laser mode with a small birefringenceinduced by the residual strain along that crystal axis.

The beam waist of the stimulated coherent radiation and the pumping beamwaist were located at the input face of crystal 11, with an accuracy ofabout ± 0.3 mm. The radius of the waist of the stimulated radiation atthe i/e² power points, w₀ was controlled by adjustment of the mirrorspacing. The value of the radius of the beam waist was inferred from ameasurement of the corresponding beam radius w₁ at the output mirror 13.The two radii are related as follows: ##EQU1## From such a measurement,I inferred that w₀ ˜ 80μ for the 50 mW threshold for oscillation, asmeasured in some of my experiments. Thus, a pump intensity, I ˜ 0.05/(π×80×10⁻ ⁴)² = 250 W/cm² is implied for threshold. The single-passgain, G, can be calculated from the equation: ##EQU2## where we assumethat the pump absorption length is short relative to the crystalthickness, where λ is the laser wavelength, n the material index, η theluminescence quantum efficiency, δν the full width of the (Gaussian)luminescence band at the half power points, and E the pump photon energyin joules. When the parameters for my laser (λ=2.7×10 ⁻ ⁴ cm, n=1.5, η≅0.4, δν= 1.5×10¹³ /sec, E/e=1.91 eV) are inserted into Eq. (2), weobtain G = 1.026 for I = 250 W/cm² ; this compares very well with therequired minimum excess gain of 2.5% per pass, if output mirrortransmission represents the only significant loss. However, since w₁(and hence w₀) is hard to determine with great accuracy, the agreementmay be somewhat fortuitous.

Tuning was accomplished in two different ways. In the first attempt, a 1mm thick sapphire plate was inserted into the cavity between the crystaland the output mirror. The c axis of the sapphire lay in the plane ofthe plate, and the plate could be rotated about its normal axis. Thus,the plate formed a birefringence tuner similar to that now used in somedye lasers. Unfortunately, since the light passing through the sapphirewas not highly parallel, the plate produced considerable astigmaticloss, and operation of the laser was difficult. Nevertheless, we wereable to tune the laser from band center at 2.68 μm to ˜2.6 μm with thisscheme.

The external grating 26 shown in FIG. 2 was somewhat more satisfactory.The grating (189 grooves/mm, blazed for 2.8 μm) acted to enhance theeffective reflectivity of the output mirror over a narrow band ofwavelengths. As the grating was rotated into adjustment about an axisparallel to its grooves and perpendicular to the laser axis, laseraction would suddenly cease at the band center, and would just assuddenly appear at the wavelength determined by the grating. The laseroutput wavelength λ, was estimated roughly by measuring the angle of the1^(st) order beam, and was measured more accurately by sending the0^(th) order beam through a Spex Model 1702, 3/4 m grating spectrometer.With this arrangement, I was able to tune continuously over the range2.576 ≦ λ ≦ 2.7850 μm. Outside this range the grating lost control ofthe laser primarily because of the reflection and absorption losses ofthe infrasil mirror substrate and infrasil lens. On the other hand,sufficient excess gain was available to have permitted laser action atwavelengths well outside the half-power points of the band. Thus, for animproved cavity and tuner arrangement, it is reasonable to predict laseraction at least over the range 2.5 ≲ λ ≲ 2.9 μm.

It is especially significant that I find no apparent bleaching or otherdamaging effects in the crystal even after many hours of continuousoperation. Furthermore, the method of tuning has verified that theluminescence band broadening mechanism is indeed homogeneous, therebyimplying the possibility of efficient energy conversion. Thus, the morepessimistic assessments of prior workers have been rebutted.

It is believed that the solid-state contact cooling illustrated in moredetail in FIGS. 2A and 2B contributed substantially to the very broadusable bandwidth and low pumping power requirements of the laser of myinvention. This mounting avoided the degrading effects of coolant fluidsflowing in the optical path and at the same time permitted the crystalto be free of birefringence that would otherwise be induced in thecrystal by differential thermal contraction.

The degrading effects of a coolant flow past the active crystal areactually two-fold, one direct and one more indirect. First, the fluidmotion of the coolant introduces index fluctuations in the optical pathfor the stimulated radiation. While such fluctuations are tolerated forthe active fluid motion of some dye lasers, as a necessary evilattendant to the need to move damaged or bleached dye out of the cavity,the problem has in fact been very discouraging to the development ofF-center lasers because of the degree of index fluctuations (bubbles) incoolant fluids. Secondly, it is difficult to maintain a coolant flow orbath of liquid nitrogen or other coolant indefinitely. When the coolantdisappears, the atmosphere envelops the crystal; and atmospheric watervapor starts dissolving and degrading the surface of the crystal. Infact, any water vapor in the optical path of the laser greatly increasesits losses, because of O-H ion absorption of the radiation. With theseproblems, other workers could not see long-term significance for theF-center laser.

In contrast, the vacuum about crystal 11 in my arrangement of FIG. 1 isrelatively easy to maintain once established, especially with the aid ofmolecular sieves. Further, no index fluctuations occur in the opticalpath; and no inherent damage of the active crystal has been discovered.Reduced optical loss due to better optical quality of the crystal yieldslower pumping threshold. All these advantages are directly attributableto the solid-state cooling.

In the side view of the cold finger 14 of FIG. 2A, it is seen that thecrystal 11 was held against the cold finger 14 at Brewster's angledirectly over an aperture 31 which is drilled through the cold finger 14in a direction suitable for passing the laser beam. The slip-fitmounting included the weak spring clip 32 and the opposed rigidretaining clip 33 which in the indicated view of FIG. 2B is seen to havehad a V-shape appropriate for retaining one corner of the crystal 11with respect to lateral movements in any direction except toward springclip 32. In the view of FIG. 2B, it is also seen that spring clip 32 hada U-shaped design so that the pumping beam may enter crystal 11 inalignment with aperture 31 without interference by spring clip 32.

As the crystal 11 contracted twice as fast as cold finger 14 astemperature fell in response to the effect of the coolant, the surfaceof crystall 11 moved over the contacting surface of cold finger 14 atall points of contact in the general direction of aperture 31. Astemperature rose, the reverse movement occurred.

Of course, it should be apparent that not all solid-state cooling of anF_(A) (II) center laser will require such relative movement.

For example, in reference to FIG. 3, if the active medium 40 isdeposited on a similar salt crystal substrate 41, it need have nosignificant differences in thermal expansion. For example, the substratecrystal 41 could be an alkali halide such as sodium chloride in whichlithium does not form color centers like those in potassium chloride.Then the entire assembly may be processed in the same way and lithiumdrifted throughout both parts without affecting either the guidingproperties of the active region 40 or its capability for stimulatedemission as compared to substrate 41. In fact, the ends of the region 40may be made highly reflective for example, ends 34 and 43, in order toform a resonator. In that event the pumping laser light from lasersource 48, for example, a krypton xenon ion laser at 6471 A, may beintroduced into region 40 through a coupling of prism 36, e.g., a rutile(TiO₂) prism, according to the well-known prism-film coupling technique.This technique is disclosed in U.S. Pat. No. 3,584,230 issued June 8,1971.

The operation of the laser may be explained in more detail withreference to FIG. 4 as follows:

The F_(A) (II) center in KCl:Li consists of an electron trapped at ananion vacancy, where one of the six nearest neighbor metal ions is anLi⁺. FIG. 4 shows an energy level diagram. The absorption band containstwo broad peaks (F_(A1) and F_(A2) designated diagrammatically by arrows51 and 52), and spans most of the red to yellow-green range of thevisible spectrum. After absorption, the F_(A) center quickly relaxes vianonradiative transitions indicated by arrows 53 and 54, to a newconfiguration whose energy levels are shown in the right hand column,and luminescent emission takes place in a band about 0.06 eV wide at thehalf-power points centered at about 2.68 μ. At liquid He temperatures,the luminescence has a decay time of 80 nsec and a quantum efficiency of40%; quantum efficiency declines slowly with increasing temperatureuntil it approaches zero at room temperature. Since the terminal stateof the luminescence is very short lived (˜10⁻ ¹² sec), the F_(A) (II)center constitutes a nearly ideal 4 level system. Quantum efficienciesapproaching unity can be expected eventually. They will providelight-to-light energy conversion efficiencies approaching 25%.

Laser action was also obtained for crystal temperatures well above 77°K.In particular, satisfactory operation was obtained for a cold-fingertemperature of ˜190°K, although the threshold was elevated to a value ˜3times as great as that obtained at 77°K; this result is consistent withthe known decrease in η with increasing temperature, and with therelative threshold measurements of Fritz and Menke. I made no seriousattempt to measure laser threshold as a function of temperature becauseof the rather poor thermal contact between the laser crystal and thecold finger. I mention this result primarily to emphasize that practicallaser operation need not be limited by the availability of liquidnitrogen; operation with dry ice or a small thermoelectric cooler shouldalso be possible.

The cavity configuration shown in FIG. 1 has two severe limitations.First, as w₀ is decreased much below 80 μm, astigmatism contributed bythe crystal begins to introduce severe losses; hence the minimum cavityspot size is severely limited. Secondly, a birefringence plate or etaloncannot be introduced into the cavity without incurring severe losses.Fortunately, highly efficient cavity designs for dye lasers employingthree mirrors are now known that allow one to substantially overcomeboth defects. For example, see U.S. Pat. No. 3,731,224, issued May 1,1973, for the invention of A. Dienes et al. With such a folded cavity,it should be possible to reduce w₀ to a value on the order of 10 μm;then by extrapolation from the present result, the pump required forthreshold would be on the order 0f 1 mW. Of course, further reduction inthreshold power should be possible through a decrease in output mirrortransmission. Hence, efficient operation with a 5 mW He-Ne laser as thepump source is quite plausible. The only practical difficulty might bewith the surface quality that is attainable in the polishing of alkalihalides. At present, I am using Linde B and pure ethanol on awool-velvet lap in the last polishing process; the resultant surface canbe made essentially scratch free. A suggested detailed polishingtechnique is that disclosed in U.S. Pat. No. 3,587,196, issued June 28,1971, for the invention of Frank A. Dunn.

Frequency definition of the F_(A) center laser may O-H be better thanthat attainable with the organic dye devices, since here there is norequired motion of the amplifying medium, and hence no associatedfrequency jitter due to index fluctuations. Other F_(A) (II) centers arealso known to exist, namely, the centers in RbCl:Li and RbBr:Li.Although the luminescence bands of these have never been accuratelymeasured, they are known to lie in the 0.4 to 0.5 eV region. The F_(A)(II) lasers may have a number of significant uses. For example, theKCl:Li laser is ideally suited for tuning through the O1H absorptionspectrum. Thus, in addition to the more ordinary chemical analyticaluses created by this 2.5-2.9 μm tuning range, the KCl:Li laser will beuniquely useful as a source for accurately measuring the O-H absorptionloss in fused-silica optical fibers. A low amount of O-H ions in thefibers is very important to their use in data processing andcommunications. Although such losses can be measured with conventionalsources in bulk material, a coherent radiation source is required tofeed sufficient light power through the fiber for adequatesignal-to-noise ratio. Mode-locked, they may provide a source ofpicosecond pulses suited to the investigation of nonlinear effects andtime-resolution studies in narrow-bandgap semiconductors. As a tunablescattering source, and perhaps in other ways, the F_(A) (II) lasersmight prove useful in the investigation of exciton charge droplets inGe, a currently interesting aspect of physics related to the phenomenaof charge distribution in solids.

My laser experiments have so far involved the F_(A) (II) centers inKCl:Li and RbCl:Li, but similar results are to be expected in a numberof other closely analogous centers, such as the recently discoveredF_(A) (II) centers in KF:Li and KF:Na, F₂ ⁺ centers in just about anyalkali halide host, and F_(B) (II) centers.

FIG. 5 shows the luminescence bands associated with some of these.Curves 61-63 represent the first three examples of the precedingparagraph. The F₂ ⁺ luminescence bands of only a few alkali halides havebeen shown in FIG. 5 in curves 64 and 65. When other hosts are included,F₂ ⁺ covers the range 0.9 ≲ λ ≲ 2 μm. If the majority of these work asexpected, color center lasers will cover the rather large and importantspectral region 0.9 μm < λ < 3.3 μm at the very least. The F_(B) (II)center curves 66 and 67 help fill in this spectrum. This region is offundamental importance to molecular spectroscopy, pollution detection,fiber optic communications, and the physics of narrow bandgapsemiconductors. In fact, it is in terms of this spectral tuning rangethat the color center lasers have their greatest advantage: organic dyesfail completely for λ ≳ 1 μm ; and the only other tunable sources in thenear infrared, the parametric oscillators, are extremely cumbersome andexpensive. Thus, color centers may indeed become a most practical anduniquely vital part of laser technology.

The energy level diagram of an F_(A) (II) center, shown in FIG. 4, maybe more completely understood from the alkali halide crystalline latticediagrams of FIGS. 6A and B. The normal configuration of FIG. 6Acorresponds to the set of levels similarly labeled in FIG. 4 on theleft. FIG. 6B corresponds to levels on the right of FIG. 4. The vacancy68 in FIG. 6A corresponds to a single "potential well"; while the twovacancies 69 and 70 in FIG. 6B correspond to a double "potential well"of the energy of the crystalline system. The small + circle is ametallic ion 71.

In contrast, relaxation of ordinary F-centers consists of a simpleexpansion of the vacancy, and of a corresponding adjustment in theelectronic wavefunction.

Nevertheless, when the ordinary F-center system reaches itsrelaxed-excited state, the associated electronic wavefunction becomesspatially very diffuse. As a result of the poor overlap between thiswavefunction and that of the ground state of the relaxed system, theoscillator strength of the luminescence band is quite small (f ˜ 0.01).Also, the relaxed-excited state is energetically quite shallow, andhence there exists the possibility of self-absorption into theconduction band of the emitted photons. These two properties takentogether make rather unlikely the attainment of a net optical gain inordinary F-centers and in F_(A) (I) and F_(B) (I) centers.

In contrast, the F_(A) centers of type II relax to a double wellconfiguration shown in FIG. 6B following optical excitation. As ourexperiments have verified, the type II F_(A) centers are eminentlysuited for tunable laser action.

Note that the relaxed F_(A) (II) center, FIG. 6B, is a radicallydifferent system from its unrelaxed counterpart, FIG. 6A. The relaxedsystem is somewhat analogous to the H₂ ⁺ molecular ion (in this casewith an additional negative charge between the two attractive centers 69and 70). The wavefunctions for the excited and ground states of therelaxed center are made up of antisymmetric and symmetric combinations,respectively, of a single-well s-state. The resultant oscillatorstrength for an electric dipole transition is quite large (f ˜ 0.2 -0.35).

Two other important features of F_(A) (II) behavior are implicit inFIGS. 6A and 6B. First (in the normal configuration), the presence of aforeign ion 71 causes p_(z) orbitals to be distinguished from p_(x) andp_(y), such that the absorption exhibits a characteristic splitting.That is, transitions to the p_(z) orbitals result in a longer wavelengthband that is often well resolved from the main band. As far as laserapplications are concerned, this extra band greatly increases theprobability of overlap with a convenient pump source.

It should also be noted from FIG. 6B that there is a 50% probabilitythat the halide ion 72 separating the two wells will move into theoriginal F_(A) center vacancy 68 upon completion of the optical pumpingcycle, that is, upon returning to the normal configuration. When suchoccurs, the F_(A) center axis will have switched orientation by 90°.Such reorientation effects must be taken into account in the design ofF_(A) (II) lasers. Pumping of the longer wavelength absorption band withlight propagating along a {100} axis bleaches that band for the pumplight itself. A similar effect occurs when pumping the short wavelengthabsorption band with light polarized along a {100 } axis. This effect isavoided in FIGS. 1, 3, 10 and 11 for F_(A) (II) centers by cutting andorienting the crystal so that the plane of pump light propagation andpolarization included a {110} axis, so that both orthogonally orientednormal states topologically the same as that of FIG. 6A are pumpedequally.

The quantum efficiency, η, of F_(A) (II) center luminescence in KCl:Liis about 40% for temperatures T ≲ 77°K, and decreases slowly withincreasing temperature until it approaches zero at T ˜ 300°K.Nevertheless, laser action has been obtained with the F_(A) (II) centerin KCl:Li for T as high as 200°K. the mechanisms for the non-radiativedecay are not yet completely understood. Behavior of F_(A) (II) centersin other hosts ought to be quite similar, although η(T) has not yet beenmeasured for these.

As indicated above, another center highly suitable for laser action isthe F₂ ⁺. This center consists of a single electron (not shown on theionic diagram) shared by two adjacent vacancies 73 and 74 for which theionic configuration is shown in FIG. 7. The arrangement shown thereapplies to both emission and absorption. For the F₂ ⁺ center, relaxationconsists merely in a slight increase in separation of the two vacancies73 and 74. The corresponding Stokes shifts are then quite small, usuallyjust enough to prevent significant overlap of the absorption andemission band. This behavior contrasts sharply with that of the F_(A)(II), where as already indicated, relaxation involves both a radicalchange in ionic configuration and a Stokes shift ratio of about five toone between absorption and emission energies.

However, in their relaxed configurations, the F₂ ⁺ and F_(A) (II) arequite similar in having double vacancies, as seen by comparing FIGS. 6Band 7; and it was this analogy that first led us to consider the F₂ ⁺ asa potential laser material. The model of an H₂ ⁺ molecular ion embeddedin a dielectric continuum seems to fit the F₂ ⁺ particularly well. Goodquantitative fit has been obtained between that model and the observedspectrum in quite a number of hosts. FIG. 8 shows the F₂ ⁺ energy leveldiagram. The levels there are named after their molecular ioncounterparts.

The transition of greatest interest for lasers is that between theground and first excited states (1sσ_(g) → 2pσ_(u)), both for excitationas well as for emission. The potential tuning range (0.9 - 2 μm) citedabove was for this emission. When thus excited, quantum efficiency ofthe emission is temperature independent, and its absolute value isprobably 100%, although the absolute value has not yet been measuredexperimentally. The oscillator strength of the emission is thought to beon the order of f ˜ 0.2.

On the other hand, the higher energy emission (2pπ_(u) → 1sσ_(g)) isprobably not of much practical value for laser action. In the firstplace, its emission is seriously quenched at all but very lowtemperatures (T ≲ 50°K) by competition from the lower energy emission.Secondly, severe overlap of the higher energy pump band with that of theordinary F-center could lead to the production of F' centers, through awell-known tunneling effect. (The F' is a vacancy containing twoelectrons. Also, it is essentially impossible to have F₂ ⁺ centerswithout a substantial accompanying population of F-centers.)Unfortunately, the F' centers would absorb strongly photons of thehigher emission band.

Perhaps it should be stated explicitly that we know of no color-centerspecies foreign to the F₂ ⁺ that would absorb photons of thelower-energy emission band shown in FIG. 8. Also, from FIG. 8 it isclear that the lower energy emission should experience no problems withself-absorption upward from its higher level 86 either because it is farfrom the conduction band edge and from the next higher level 85. It isbelieved that a concentration of 1 × 10¹⁶ /cc. of F₂ ⁺ centers should beadequate for laser action, although a somewhat higher concentrationwould be preferred.

Finally, we should point out one further advantage of the F₂ ⁺ colorcenter. This advantage resides in a combination of the following: First,the proposed laser transition is strongly polarized along the axis ofthe center. Secondly, the center axes can all be aligned along oneparticular {110} axis, and optical pumping of the lower energy band willnot affect that alignment. The alignment is illustratively achievedthrough optical pumping of the parent F₂ center with linearly polarizedlight in its higher energy absorption bands. For example, the F₂ centerin KCl is aligned by pumping with polarized light of wavelength ≲ 540nm.Thus, all the F₂ ⁺ centers could contribute with maximum efficiency to alinearly polarized laser mode. When taken together with the small Stokesshift, this property could lead to an energy conversion efficiency forthe laser as high as 80%.

There is at least one more color center type that is promising for laseraction, and that is the F_(B) (II). The ionic configuration of the F_(B)(II) center is shown in FIGS. 9A and 9B. The F_(B) centers are quitesimilar to the F_(A), except that they involve two foreign metal ions 91and 92, instead of the single foreign ion of the F_(A) center. F_(B)centers are obtained in substantial quantities when the dopantconcentration represents at least several percent of all metal ions. Inthe F_(B) (II) center, note that metal ions are diagonally aligned withrespect to the vacancy or vacancies and the principal directions ofionic alignment. The two vacancies 94 and 95 yield a desirable doublewell potential characteristic. In contrast, the undesired type I F_(B)center has the foreign ions lying along a common {100 } axis, which isat 45° with respect to that shown for metal ions 91 and 92, and does notyield as desirable a characteristic.

There is a fair chance that F_(B) (II) centers will allow the lasertuning range to be pushed well into the region λ > 3μm. That is,luminescence photon energies of the type II centers seem to decreasemonotonically with increasing lattice size. However, the largest latticein which F_(A) (II) centers are known to exist is RbCl. On the otherhand, the F_(B) (II) configuration seems to allow formation in latticescontaining heavier ions. Thus, it seems rather likely that type IIcenter formation will be possible in a lattice such as RbBr or perhapseven RbI, for F_(B) centers whose normal configuration is that shown inFIGS. 9A and 9B.

One of the potential difficulties with the F_(B) (II) centers is thatthey are always accompanied by a certain population of F_(A) (I) andF_(B) (I) centers. Furthermore, the absorption bands of all three centertypes tend to overlap rather strongly. However, both the F_(A) (I) andF_(B) (I) center axis can be aligned along a common {100} axis byoptical pumping with polarized light when the crystal is at or near roomtemperature; and it is such a crystal that is used when an F_(B) (II)center laser, for example, that of FIGS. 10 and 11, is to be built. Thelong wavelength absorption band of both type I centers will then bebleached out for light polarized at right angles to that [100] axis,whereas the F_(B) (II) centers will continue to absorb for thatcombination of wavelength and polarization. Thus, selective excitationof the F_(B) (II) centers can be achieved. Not only will this allow formore efficient use of optical pump power, but it will also avoid thecreation of potentially harmful species. That is, optically excitedF_(A) (I) and F_(B) (I) centers may tend to absorb at wavelengthscorresponding to the intended F_(B) (II) laser emission.

The processes by which the above-mentioned color centers are formeddeserve mention. In all cases, ordinary F-centers are created first,usually through the well-known process of additive coloration, or bysubjecting the crystal to radiation damage. In general, additivecoloration is preferred for the purity of its end product.

The formation of F_(A), F_(B), or F₂ centers results from a simpleaggregation process that can be described as follows: First, thermalionization of optically excited F-centers results in the formation ofpairs of F' centers and empty vacancies. At sufficiently hightemperatures (T ≲ - 50°C), the empty vacancies wander through thelattice until they meet either an F-center or a foreign metal ion.Recapture of an electron (from optically ionized F' centers) by thevacancy then leads to formation of F₂ centers in the first instance, orto the formation of F_(A) or F_(B) centers in the second.

If the foreign metal ion concentration is several orders of magnitudegreater than that of the F-centers, an essentially complete conversioncan be carried out, with F_(A) centers as the exclusive end product.However, the creation of F₂ centers cannot be carried to completionwithout an accompanying creation of higher aggregates, such as F₃ andF₄, which need not be explained here as to configuration. Thus, theoptimum conversion to F₂ will necessarily involve a finite residue ofF-centers.

F₂ centers are converted to F₂ ⁺ by subjecting the F₂ centers toionizing radiation. The conversion is permanent if suitable traps havebeen provided for the excess electron. A number of successful schemesfor making such traps are known.

Finally, we should mention the U center, which is an H⁺ ion trapped atan anion vacancy. U centers are usually formed by baking a crystalalready containing F-centers in an atmosphere of H₂. The U centersabsorb only in the hard U.V. and they are very stable. They can beconverted back into F-centers either by pumping them with U. V. or bygentle X-raying of the crystal. This process of temporarily "storing"F-centers as U centers is quite helpful in the manufacture of laserquality crystals.

The modified embodiment of FIG. 10 is based on the dye laser art. In cwdye lasers, the modal beam is highly concentrated in the region of thedye cell. The beam is focused by reflectors 101 and 102 to a diffractionlimited spot, called the beam waist, whose diameter is typically on theorder of 10 μm. This small spot size allows the pump beam also to behighly concentrated, such that a power of, say, 1 watt can produce apump intensity in the region of the dye cell on the order of 10⁶ w/cm².In addition to allowing for the maximum possible concentration of pumppower, such tightly focused, coaxially pumped cavities are energeticallyhighly efficient, since practically all the incident pump power can beabsorbed in a volume of the amplifying medium that is coincident withthat swept out by the laser mode itself.

Such focused-beam cavity designs are also quite suitable for use in acolor center laser. For the most part one needs merely to substitute athin slab of colored crystal 104 for the dye cell. Otherwise, thetechnology that has developed around such cavity designs can be takenover more or less wholesale. The pumping laser medium (not shown) istypically, though not necessarily, disposed between reflectors 102 and103.

In FIG. 10 the amplifying crystal 104 is located in the neighborhood ofthe beam waist, as shown. The angle φ is Brewster's angle, such thatreflection loss at the crystal surfaces (for a mode whose electric fieldis in the plane of the paper) can be avoided without the necessity forantireflection coating them; this is a big advantage for the colorcenter device, where the necessity to add antireflection coatings wouldpose difficulties in crystal handling. Another very important feature ofthis design is that the astigmatism induced by mirror 102 can be made toexactly compensate for the astigmatism created by rotation of thecrystal to Brewster's angle. This compensation can be accomplishedthrough adjustment of the reflection angle 2θ. Without thiscompensation, it would be impossible to reduce the beam waist below acertain critical size, and still maintain a stable laser mode.

The laser mode itself can be described as follows: The beam waist, ofradius W_(O), is located almost exactly at the center of curvature ofmirror 101, and just a little outside the focal length f of mirror 102.When the output mirror 103 is a flat, the modal beam has its largestdiameter (2W₁) at mirror 102, and slowly tapers down to a second beamwaist, of diameter √2 W₁, at the output mirror 103. However, since oneusually has d₂ >>> 2W₁, for all practical purposes, the mode consists ofan essentially parallel beam in the region between 102 and the outputmirror 103. This region of parallel light constitutes yet anotherimportant feature of the cavity design, since it greatly facilitates theinsertion of tuning elements and other intracavity devices.

where

     )

The cavity of FIG. 10 has been described quantitatively in great detailby Dienes et al. in U.S. Pat. No. 3,731,224, cited above. The mostimportant results of their paper can be summarized as follows: The modeis said to be stable as long as W₁ is of finite size. Let us define δsuch that

    d.sub.1 ≡ r.sub.l + f + δ                      (3)

where r₁ is the radius of curvature of 101. A stable mode will beobtained as long as δ lies within the range

    0 ≲ δ ≲ f.sup.2 /d.sub.2 ≡ 2S. (4)

the quantity of 2S is known as the stability range. The confocalparameter, b, is defined as the distance between points along the beampath where the mode diameter is √2 times that obtaining at the beamwaist. One always has

    b = 2πW.sub.O.sup.2 /λ.                          (5)

when the cavity is adjusted to the middle of its stability range, onealso has

    b ≅ f.sup.2 /d.sub.2 = 2S.                       (6)

by combining Equations (5) and (6), one can calculate W_(O) : ##EQU3##Yet another important quantity is the far-field angle φ, which can becalculated as ##EQU4## and from which W₁ can be calculated as

    W.sub.1 ≅ fφ.                                (9)

Finally, the astigmatic compensation mentioned above will be obtainedwhenever the relation

    f sin θ tan θ = t(n.sup.2 -1) √n.sup.2 +1/n.sup.4 (10) is satisfied, and where t and n are the crystal thickness and index of refraction, respectively.

Kogelnik et al. also discuss the nature of the mode shape inside theamplifying medium. Although in general the behavior is quitecomplicated, they find that for the case S >> t, the beam waist area Ais given by the expression

    A ≅ πnW.sub.O.sup.2                           (11)

whereas for the case S << t, they obtain

    A ≅ λt(n.sup.2 -1)/[n.sup.3 √n.sup.3 +l]. (12)

In this section we shall develop some useful formulas for the opticalgain, or amplification that can be attained in a four-level system suchas the F and F-like centers. Not only will these formulas prove usefulin the description of working lasers, but also they will allow us toconclude the general discussion of the suitability of various centertypes for practical laser action.

As already indicated above, the optical pumping cycle of F and F-likecenters consists of four steps: excitation, relaxation, luminescence,and relaxation back to the ground state of the normal configuration (seeFIG. 2). Since both relaxation times τ_(R) and τ_(R') are very short(τ_(R) and τ_(R') ≲ 10 ⁻ ¹² sec.) with respect to the luminescence decaytime τ_(l), the only populations of any significance are N and N*, i.e.,those of the ground and relaxed-excited states, respectively. Thus,population inversion of the luminescence levels is obtained at anyfinite pump rate, and in this limited sense, any F or F-like centerforms an ideal four-level system.

But one must also consider the possibility of absorption at theluminescence frequency. That is, in the Beer's law expression for thenet optical gain:

    G ≡ I.sub.out /I.sub.in = e.sup.α.sup.z        (13)

where z is the gain path length, the net gain coefficient is the sum oftwo terms,

    α = α.sub.g - α.sub.l                    (14)

where α_(g) is the coefficient calculated for the inverted luminescencelevels alone, and where α_(l) is the coefficient of absorption loss. Ofcourse, only when α_(g) > α_(l) can there be a net gain.

Even when the unexcited crystal is perfectly transparent at theluminescence frequency, there sometimes exists the possibility that theoptical pumping will create a new and absorbing species. Such occurs,for example, with those centers retaining the vacancy configuration inthe relaxed-excited state, such as the F and F_(A) (I). As alreadyindicated hereinbefore, one new species is the relaxed-excited stateitself, since absorption at the luminescence frequency from therelaxed-excited state into the conduction band is a distinctpossibility. Such absorption from the relaxed-excited state of theordinary F center has been measured for slightly lower photon energies,and it has been found to be quite large. For the case of opticallypumped F and F_(A) and F_(A) (I) centers, whenever N ≳ 10¹⁶ - 10¹⁷ /cm³,F' centers are formed in significant quantities. These constitute yetanother absorbing species, since the F' absorption band severelyoverlaps the F luminescence band.

In contrast, the above problems are essentially nonexistent for centershaving a double well relaxed-excited state. In the first place, theseparation between the relaxed-excited state and the lower edge of theconduction band is greater than the luminescence photon energy. Also,there is no overlap between the F' absorption and the luminescence band.With a tunable laser using the F_(A) (II) center in KCl:Li, we have onoccasion seen evidence for an absorption at the extreme long wavelengthedge of the luminescence band, but this mysterious absorption can beeliminated through proper treatment of the crystal.

For a Gaussian band of full width at the half power points δν, the gaincoefficient at the band peak, α_(o), can be calculated from thewell-known formula ##EQU5## where λ_(o) is the wavelength at the bandcenter, n is the host index, η is the quantum efficiency ofluminescence, and τ_(l) is the measured luminescence decay time.

The gain formula (15) represents an extremely useful form, and one thataffords maximum insight. In the first place, it contains onlyexperimentally measurable quantities, with the sole exception of N*.Furthermore, in general, N* is not limited by the pump, since extremelyhigh intensity sources are now available. Instead, the limitation in N*,if any, is created by other factors, such as a rapid increase ofinteraction among centers with increasing concentration. From this pointof view, it is more revealing to display N* directly, rather than toexpress it in terms of a pump intensity.

In keeping with the above philosophy, let us compare the gaincoefficients possible with various color center types for a fixed valueof N*. In Table I we show values of α_(o) calculated from Equation (15)for the F and F₂ ⁺ centers in KCl, and for the F_(A) (II) center inKCl:Li. In each case, n = 1.49 and T = 77°K. Note that α_(o) is twoorders of magnitude smaller for the F center than for the other centers.This difference reflects the combined effects of an approximately 22times smaller oscillator strength and a roughly 4 times larger δν forthe F center.

The gain figure of Table I for the ordinary F center is ominously smallin absolute magnitude. In the first place, only a rather smallabsorption would bring the net gain to zero; several possible sources ofsuch absorption have already been pointed out. In the second place, sucha small gain coefficient tends to rule out the possibility of using acoaxial pumping scheme, where the effective gain path length is thereciprocal of the pump band absorption coefficient. That is, for Fcenters pumped somewhere near the absorption band peak, the gain pathwould always be too short to allow for a reasonable gain.

Although it is not as fundamental as Equation (15), we shall have usefor an equation that expresses the gain as a function of pump intensity.Since the pump rate out of the ground state is equal to the photonabsorption rate, we may write ##EQU6## where W is the pump rate, β isthe absorption coefficient at the pump wavelength, E_(p) is the pumpphoton energy, and I is the pump beam intensity. We may then writefurther: ##EQU7## Finally, substituting (17) into (15) we obtain,##EQU8##

For a coaxially pumped system, where one can assume a variation of thegain coefficient of the form

    α = α.sub.0 e.sup.-.sup.β.sup.z           (19)

and if furthermore, the gain path is long with respect to β⁻ ¹, then itis easy to show that

    G = e.sup.α.sbsp.0.sup./ .sup.β.                (20)

Equations (18) and (19) may be combined to yield ##EQU9##

                  TABLE I                                                         ______________________________________                                        QUANTITY F         F.sub.2.sup.+                                                                           F.sub.A (II)                                                                          UNITS                                    ______________________________________                                        λ.sub.o                                                                         1.0       1.68      2.7     μm                                    τ.sub.l/.sub.η                                                                 600       200       200     nsec                                     δν                                                                            6.3       1.69      1.45    10.sup.13 Hz                             α.sub.o                                                                          0.004     3.5       4.2     cm.sup.-.sup.1                           ______________________________________                                    

In FIG. 10 and also in FIG. 11, the basic cavity configuration is thatdescribed in the preceding paragraphs, with the following parameters:radius of curvature of reflector 102 or 112 and 101 or 111, distancefrom crystal 104 = 25mm, d₂ = 600mm, thickness of crystal 104 = 1.72mm,and 2θ = 20°.

For operation with the F_(A) (II) centers of KCl:Li and RbCl:Li, thepump source was a krypton ion laser 110 operating at 6471 A. Both inputand output mirrors 111 and 113 were of the multilayer dielectric type.Thus, it was possible for the input mirror to be a high reflector (R ˜100%) for the wavelengths in the λ = 2.7 μm band, and at the same timeto have rather high transmission at the pump wavelength. Theintermediate mirror 112 was coated with evaporated gold. A simple lens115 located immediately behind reflector 111 served to bring the pumpbeam to a focus at the crystal 104. Actually, it was also necessary touse a second, very weak lens located just outside the pump beam inputwindow 116 in order to achieve perfect focus, but this allowed for veryconvenient external adjustment of both the pump beam position as well asits focus.

The crystal slab 104 was held against a Cu cold finger with a gentlespring clamp, as shown in FIGS. 2A and 2B for crystal 11. No grease orother thermally conductive compound was used, in order not to strain orfracture the crystal by differential thermal contraction. Despite therather poor thermal contact between the crystal and cold finger, cw pumpinputs as high as ˜200 mw could be tolerated without undue heating ofthe crystal.

For an F_(A) (II) center laser, the two opposing large faces of thecrystal 104 were {100} planes. The crystals were oriented such that theplane of the paper in FIGS. 10 and 11 contained a 110 axis. Since thepump beam was also polarized in this plane, both sets of centers whoseaxes lay in the plane of the slab were pumped equally. Also, due to theBrewster's angle orientation, the laser beam traversed the crystal at aconsiderable angle (36°) to the slab normal, and hence centers whoseaxes lay along that direction were also pumped. In this way it waspossible to avoid bleaching the crystal, even though the long wavelengthabsorption band was used for pumping. A second advantage of the {110}orientation was that it prevented strain-induced birefringence fromaffecting the laser mode, since such strains tended to lie along the{110} axis.

The laser crystals were prepared as follows: Slabs 104 of the desiredthickness were cleaved out of a crystal containing about 10¹⁸ Ucenters/cm³. The two opposing large faces of the crystal were thenoptically polished with Linde B and pure ethanol on a wool velvet lap.The transparency of the U center crystal greatly facilitated inspectionfor surface flaws, strain-induced birefringence, and other possibledefects. A fraction of the U centers were then converted into ordinary Fcenters by subjecting the crystal for about 10 min. to 50 kv X-rays withbeam current of 17 ma, crystal located ˜ 2 cm from the anode. Theresultant F center concentration was ˜ 1 - 2×10¹⁷ /cm³. Immediatelyfollowing X-ray treatment, the crystal was loaded into the laser vacuumchamber 117 and cooled to about -30°C. The F centers were then convertedinto F_(A) (II) centers by optically pumping the F band for nearly onehour or more. It was found necessary to obtain very complete conversion,since optically excited F centers are rather strong absorbers in theF_(A) (II) luminescence band. Finally, the crystal was cooled to 77°Kfor laser operation.

Tuning was illustratively accomplished with a set of sapphirebirefringence plates 118A-C, as shown in FIG. 11. These plates wereoriented at Brewster's angle with respect to the beam. The optic axis ofeach lay in the plane of the plate. A simple mechanical linkage (notshown) allowed all three plates to be rotated simultaneously about theirnormal axes. In this way it was possible to tune the KCl:Li F_(A) (II)laser smoothly and continuously over the full range 2.5 ≦ λ ≦ 2.9 μmwith rotation of a single knob.

Space does not permit an extensive discussion here of the birefringencetuner 118A-C, but a rather complete description has been given recentlyin the literature in the article by A. L. Bloom, Journal of the OpticalSociety of America, 64, 447 (1974). The essential principle of the tuneris this: in general, a linearly polarized mode is made ellipticallypolarized by the plates 118A-C, and a large reflection loss results.However, there exists one wavelength for which the linear modepolarization is unaffected, and there is no loss; laser action thenoccurs at this favored wavelength. One can easily show that the lasingwavelength varies with plate rotation angle θ₀ as

    λ = λ.sub.p (1-cos.sup.2 φcos.sup.2 θ.sub.0)(22)

where φ is Brewster's angle and where λ_(p) is a constant determined bythe plate thickness and birefringence constants. For sapphire, φ = 60°,or cos² φ = 0.25, such that a practical tuning range a little less than25 percent wide is possible with a single set of plates 118A-C.

The entire cavity 111, 112, 113 was preferably surrounded by a vacuumenclosure 117, as indicated by the dashed lines of FIG. 11. The vacuumwas employed for two reasons, first, to provide thermal insulation forthe crystal, and secondly, to prevent atmospheric absorption (especiallyfrom H₂ O) from interfering with laser operation. The cylindrical can119 surrounding the crystal 104 and spherical mirror section 111, 112was open at the top (toward the viewer), and the vacuum seal wascompleted by a removable liquid nitrogen storage can. The output mirror113 was mounted on a flexible metal bellows 120, to allow for angularadjustment during laser operation, but the two spherical mirrors 111 and112 had to be prealigned.

The cavity was prealigned with the aid of a λ = 5682A krypton ion laserline from source 110, suitably adapted, for which wavelength the mirrorswere rather good reflectors. A dummy (transparent) crystal 104' (notshown) of like dimensions and index to crystal 104 was inserted into thecavity for this purpose. With the aid of this visible light, the limitsof the cavity stability range could be found easily and rapidly, and themirrors 111, 112 and 114 set into proper angular adjustment. Usually nofurther adjustment was required for obtaining laser action after suchprealignment, except perhaps for a slight touch-up of the output mirror113 and pump beam input steering lens 116A.

In order to measure the gain capabilities of the KCl:Li laser, the usualhigh-reflectivity output mirror 113 was replaced with one having R =50%. In this way the unknown intracavity losses would be overwhelmed bythe huge output transmission loss. In this experiment, threshold forlaser action (at band center) occurred at an input pump power, P, to thecrystal of 130 mw. At threshold, the gain (for a double pass through thecrystal) just compensates for the total cavity loss. Thus a single passgain of √2 = exp(0.346) was implied. This measured gain is to bbecompared with that calculable from Equation (6).

To make that calculation, first we must estimate the pump intensity, I.From Equations (6) and (7) and the cavity parameters listed above, weobtain b = 1.02mm and W₀ ≅ 20 μm. Since the conditions for Equation (12)are better satisfied than those for Equation (11), we estimate the modebeam cross section from Equation (12) as A = 0.83 10⁻ ⁵ cm². By makingthe further assumption that pump beam was well matched in size to themodal beam, we may estimate the pump intensity I from the modal area andthe threshold pump power of 130 mw. Then from Equation (2), we finallycalculate ln G = 0.39, in excellent agreement with the measured value.

When a high reflectivity output mirror 113 (T = 1.6%) was used, the pumppower at threshold dropped to 14 mw. Since ln G is directly proportionalto the pump power, a value of ln G = 0.037 is implied by extrapolationfrom the previous experiment. Hence the total cavity loss was 2×0.037 =7.4%, of which 1.6% represents output coupling, and the remaining 4.8%is the intracavity loss. Since we were not able to measure theindividual mirror reflectivities, an exact accounting of the intracavityloss is impossible.

The maximum energetic efficiency of the KCl:Li laser should be about10%, since the ratio of pump to luminescence photon energies is 5, andonly about one-half the centers will be oriented such that they canradiate into a linearly polarized laser mode. This figure must befurther multiplied by the ratio of output mirror transmission to totalcavity loss. Thus, with T = 1.6% and the loss of 7.4% cited above, ourlaser should have had an energetic efficiency, when operated far abovethreshold, of approximately 0.1×1.6/7.4 = 2.2%.

FIG. 12 contains curve 121, a graph of laser output power as a functionof pump input for operation with the 1.6% transmission output mirror113. The behavior shown there is consistent with the above efficiencyestimate.

In FIG. 13, the reciprocal of the pump power required at threshold,P_(th).sup.⁻¹, is plotted in curve 122A as a function of laserwavelength. Ideally, this curve 122A should have the same shape as theluminescence band, curve 122B. However, the deviation between the twocurves indicates the existence of an absorption in the crystal 104 thatincreases in strength with increasing wavelength. In fact, the laseroften could not be made to function for λ ≳ 2.8 μm, and the data of FIG.13 represent the best attainable behavior. The improved performancecould always be attained by heating the crystal to T = -30°C, andintensely pumping it for 20 or 30 minutes before cooling it down againfor laser operation. It would be most interesting and helpful if theorigin of this sporadic and annoying absorption could be identified.

In FIG. 14, curve 123 gives some indication of the spectral purity ofthe laser output. We have chosen to illustrate the worst case, i.e. thatobtaining when the laser is operated far above threshold. Each of thetwo peaks in FIG. 14 probably represents the response of the gratinginstrument to a δ function in frequency; hence, we probably havesimultaneous operation on two closely spaced mode frequencies. It may bethat true single frequency operation can be attained through bettermutual adjustment of the sapphire birefringence plates, but if not, suchsingle frequency operation can always be attained through addition of anintracavity etalon.

Despite the early stage of development, the performance of the KCl:Lilaser just described is quite promising. More efficient mirrors and asomewhat higher output mirror transmission would greatly increase itsenergetic efficiency. Tuning has been accomplished over the full-rangelying between the 25% power points of the luminescence band; this is notalways possible with organic dyes. And with its static amplifyingmedium, crystal 104, as opposed to a rapidly flowing and turbulent dyesolution, the color center device of FIG. 11 is inherently capable ofmuch better frequency definition and stability.

Although we have spoken mainly of KCl:Li in the above account of laserexperiments, it should be mentioned that we have also obtained quitesatisfactory operation with RbCl:Li as well. At the time of thiswriting, experiments to test the F₂ ⁺ as a laser material are in thepreparation stage. The host lattice will be KCl with F₂ ⁺ centers,described hereinbelow, for the first test. The pump source 110 will be aNd:YAG laser operating at 1.34 μm. The cavity configuration will beessentially that just described for the F_(A) (II) center device of FIG.11. In other words, FIG. 11 is modified as just described for mypreferred form of F₂ ⁺ laser.

Among the modifications that are desirable for the foregoing embodimentsfor particular applications are almost any of the optical resonatorconfigurations currently usable with dye lasers operating in the visibleportion of the spectrum. For example, highly efficient three-reflectorresonators are available to overcome the astigmatism associated with ahigh degree of focusing of the pumping and stimulated beams in thecrystal. Such resonators also reduce loss associated with theintroduction of a birefringent plate or etalon for tuning. Thus, thediffraction grating can be eliminated.

For certain applications, such as pollution monitoring, it would bedesirable to have a very inexpensive laser that would be fixed-tuned toa predetermined frequency, such as a prominent absorption line of agiven molecular species. FIG. 15 suggests one way that such a devicemight be made from color centers. It would make use of the principle ofdistributed feedback. That is, if either the index n or the gaincoefficient α is modulated spatially with period d, there will be strongfeedback at those wavelengths that satisfy the Bragg condition:

    nλ = 2d                                             (23)

without the need for external mirrors.

The device shown in FIG. 15 would use a modulation of α itself, obtainedby means of a periodic variation in the F_(A) center concentration. Therequired "grating" could be written into the KCl:Li or other similarcrystal 131 by taking advantage of the photochromic conversion process U→ F and subsequent conversion to F_(A) (II) in regions 134. The beam ofan ultraviolet laser could be split and made to interfere with itself toform an interference pattern of the desired period. Regions 134 would bedefined by regions of highest radiation intensity in the forminginterference pattern. Remaining U regions 135 would occur at regions ofdestructive interference or radiation nulls. The exposure technique is awell-known one that has been used rather extensively to write gratingsinto certain photochromic plastic materials.

The principle of distributed feedback has been given rather extensivetreatment by Kogelnik et al. in their U.S. Pat. No. 3,760,292, abovecited. They show that the product αl must be of the order unity, where lis the total gain pathlength in order to have oscillation when pure gainmodulation is used. For l on the order of a few cm, the pump intensityrequired would be on the order of 30 - 100 w/cm². Such intensities oughtto be obtainable from an arc lamp, at least on a pulsed basis. For thedesired positive distributed feedback each pair of regions 134 and 135has a combined width equal to an odd multiple of half-wavelengths, whereλ is the wavelength of light to be stimulated.

Further, the relatively high concentration of F_(A) (II) centers inregions 134 and the degree of focusing of the pumping light from souce136 by lens 133 can be considerably relaxed in the embodiment of FIG. 3,as compared to the relatively high degree of focusing in FIG. 1, as theguiding pathlength l; orthogonal to stripe-like regions 134 and 135 incrystal 131 is lengthened.

The above-described arrangement is illustrative of the application ofthe principles of the invention. Other embodiments may be devised bythose skilled in the art without departing from the spirit and scopethereof.

For example, I now appreciate that principal features of my inventionare potentially applicable to a broad class of F-type centers, otherthan ordinary F-centers. In particular, in addition to the F_(A) (II),F_(B) (II) and F₂ ⁺ center lasers disclosed and claimed above, it is nowfelt that an F₂ center laser may be feasible if further investigationshows that an active medium including such centers has a sufficientlylow self-absorption for the stimulated radiation.

I claim:
 1. Apparatus for the stimulated emission of radiation of thetype comprising a body of an alkali halide crystal having color centerscapable of substantial absorption of a particular band of radiation,means for supplying the band of radiation to the crystal to produce apopulation inversion between radiatively coupled energy levels, andmeans including reflective surfaces associated with the crystal forstimulating the emission of coherent radiation from the invertedpopulation of color centers, characterized in that the apparatusincludes means for cooling the crystal by conduction from the crystal,said cooling means including a substantial solid body contacting saidcrystal and providing an optical path within said body of negligibleloss for the stimulated radiation.
 2. Apparatus according to claim 1 inwhich the body of an alkali halide is a body of potassium chloride(KCl), said body containing at least 1 × 10¹⁷ F_(A) (II) color centersper cubic centimeter of the type associated with a lithium ion as one ofthe six nearest neighbors.
 3. Apparatus according to claim 1 in whichthe solid body contacting the crystal comprises a metallic heat sink incontact with a major surface of said body, said heat sink having anopening therethrough in alignment with a direction of Brewster-anglepropagation of the stimulated radiation in said body, and meansincluding a spring clip for providing a centered slip-fit mounting ofsaid body on said heat sink.
 4. Apparatus according to claim 3 in whichthe cooing means includes a vacuum chamber, the body of potassiumchloride being mounted on the metallic heat sink within said vacuumchamber.
 5. Apparatus according to claim 1 in which the body of analkali halide is a fiber-like body disposed on a dielectric substratenot containing color centers competitive with those in the fiber-likebody.
 6. Apparatus according to claim 5 in which the body of alkalihalide has predominantly F_(A) (II) centers caused by either of theimpurities Li⁺ or Na⁺.
 7. Apparatus of the type claimed in claim 1 inwhich the body of alkali halide has at least one crystalline {110} axisand has predominantly F₂ ⁺ centers having all their axes aligned alongsaid one {110} axis of said body, and in which the means for pumping thebody comprises a source of radiant energy substantially completely ofphoton energy no greater than that within the lowest energy absorptionband of said F₂ ⁺ centers.
 8. Apparatus of the type claimed in claim 1in which the body of alkali halide has in addition to the selected colorcenters the potentially interfering F_(A) (I) and F_(B) (I) centers andhas a substantial concentration, greater than 1 × 10¹⁸ per cubiccentimeter, of the selected F_(B) (II) centers that predominate oversaid F_(A) (I) and F_(B) (I) centers, said body having at least a {100}axis along which said F_(A) (I) and F_(B) (I) centers are aligned, andin which the means for pumping the body comprises a source of radiantenergy polarized substantially at right angles to said {100} axis. 9.Apparatus of the type claimed in claim 8 in which the body of alkalihalide is selected from the group consisting of RbBr and RbI. 10.Apparatus of the type claimed in claim 1 in which the body of alkalihalide and the means for stimulating the emission from the invertedpopulation mutually comprise alternating regions of U centers and theactive color centers, said U center regions including excess hydrogen torender the body colorless therein, said active center regions and said Ucenter regions having widths orthogonal to their interfaces equal to oddmultiples of the wavelength of said emission to provide a distributedfeedback effect.
 11. Apparatus for the stimulated emission of coherentradiation comprising a body of an alkali halide containing a substantialconcentration of color centers, said color centers being one typeselected from the group of types of color centers consisting of F_(A)(II), F₂ ⁺ and F_(B) (II), means for providing heat transfer from saidbody primarily by solid-state contact, means for exciting said body toprovide a population inversion therein by absorption by said selectedcolor centers including means for pumping said body with radiant energypredominantly within the absorption bands of said selected colorcenters, and means for variably resonating emission from the invertedpopulation to stimulate the coherent radiation.
 12. Apparatus of thetype claimed in claim 11 in which the body of alkali halide is orientedto have a {110} axis at an acute angle with respect to the pump beamdirection and lying in the plane defined by the pump beam direction andpolarization to pump equally centers of one type and two orthogonalorientations.
 13. Apparatus for the stimulated emission of coherentradiation comprising a body of potassium chloride containing lithium asan impurity, said body containing at least 1 × 10¹⁷ color centers percubic centimeter of type F_(A) (II), means for cooling said body,including a copper heat sink and means for providing a slip-fit mountingof said body to said heat sink with its thinnest dimension being in adirection at Brewster's angle with respect to the laser axis, means forcontinuously pumping said body in its red absorption band attributableto F_(A) (II) centers to provide a population inversion in said body,and means for stimulating the emission of coherent radiation from saidbody, including a resonator including first and second nearlyconcentrically-spaced focusing reflectors the first of which partiallytransmits the pump light and the second of which partially transmits theinfrared light, and disposed beyond the second of said reflectors, afocusing lens and a diffraction grating rotatable about an axisorthogonal to the laser axis and parallel to the grating plane topromote adjustment of the oscillation frequency.
 14. An optical deviceof the type comprising a body of an alkali halide having active colorcenters therein and means for supplying light of well-defined frequencyto said body to yield light of another well-defined frequency, saiddevice being characterized by alternating regions of U centers and theactive color centers, said U center regions including excess hydrogen torender the body colorless therein, said active center regions and said Ucenter regions having widths orthogonal to their interfaces equal to oddmultiples of the wavelength of said emission to provide a distributedfeedback effect.